What is dark matter?

The year was 1925, and astronomy was again on the verge of great discovery. Edwin Hubble had confirmed the existence of other galaxies, proving space to be more vast than ever imagined. As the known universe expanded, so did curiosity about its secrets. A new world awaited exploration.

Astronomers studied these new galaxies, observing their structure and motion. They existed on scales greater than scientists had ever conceived. Many galaxies appeared in elliptical and spiral shapes orbiting supermassive centers. When researchers recorded their velocities, however, they came upon a conundrum: they were spinning incorrectly.

A photo of the Andromeda galaxy from 1899. The galaxy was at this point thought to be part of the Milky Way. That or some large camera smudge that wouldn’t go away.

As an object moves toward a massive body, gravity’s pull becomes stronger. Objects close to a source of gravity must move quickly to avoid falling into it, while objects further away orbit more slowly. This meant that stars near the centers of these galaxies would travel at huge speeds compared to those on the outskirts. Even so, measurements showed that the stars on the edges of the new galaxies were moving at the same high speeds—too high, in fact, to stay in orbit. According to the equations, the stars were travelling far above escape velocity and should have flown off into space.

The only explanation for these orbits was that these galaxies were more massive than they appeared. They contained something beyond what we could see, some sort of dark matter distributed evenly enough to keep gravity relatively constant throughout. This mysterious substance has become one of the greatest mysteries of modern science. Unlike regular atoms and molecules, dark matter does not interact with light. It is invisible, measurable through only its gravitational force. Even stranger, astrophysicists calculate that dark matter makes up 23 percent of matter and energy in the universe while normal visible matter composes just 4.6 percent (the remaining 72 percent is dark energy, but that’s for another post!).

What is dark matter? Simply put, we don’t know. Early theories focused on massive astrophysical compact halo objects, commonly referred to as MACHOs. Proponents hypothesized that large objects such as very dim brown dwarfs or rogue planets could account for galaxies’ missing mass, increasing their gravity while remaining invisible to observation. Calculations show, however, that not enough of these objects exist to account for all dark matter, which has led astronomers to abandon MACHOs in favor of WIMPs.

An artist’s impression of the dark matter surrounding the Milky Way. The galaxy has filed complaints against this image, claiming that it makes it look fat.

Weakly interacting massive particles or WIMPs are sub-atomic particles that do not absorb or emit light. These hypothetical particles interact through only gravity and the weak nuclear force, making them extremely difficult to detect. Astrophysicists believe that dense collections of WIMPS may account for dark matter, giving galaxies extra mass while remaining invisible to most observation. Laboratories around the world have been built to detect these and other sub-atomic particles. As of yet no team has conclusively confirmed their presence.

In 1900 Lord William Thompson Kelvin infamously proclaimed, “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.” History would of course prove him wrong. As we drift through the infinite expanse of the unknown, there is one truth we can be sure of: there is more to the universe than meets the eye.

Exploring dark matter could lead to new understandings of the universe’s most fascinating objects and help Neil deGrasse Tyson in his quest to master the force.

So what do you think about dark matter? Does it challenge the way you see the universe? Share your thoughts in the comments section below! As always, please like, share, or reblog this post if you enjoy it. Be sure to check me out on Twitter and Facebook as well. Thanks for reading! Don’t forget to subscribe for new content every Wednesday!